1. Field of the Invention
This invention relates to information storage devices and in particular to magnetic bubble devices.
2. Art Background
Extensive investigation has been applied to the preparation of information storage devices that depend on uniaxial single wall magnetic domains, i.e., magnetic bubble domains, for the storage of information. Typically, these devices include a garnet composition that is capable of supporting magnetic bubble domains through a magnetic anisotropy leading to an easy axis of magnetization perpendicular to the garnet surface. The uniaxial magnetic domains are typically produced, maintained, and propagated in the garnet material through the influence of magnetic fields. Among the fields employed is a static field that is introduced generally through the use of a barium ferrite permanent magnet which is part of the final device. This field maintains the uniaxial magnetic bubble domains once they are formed. The minimum and maximum strength of this static field necessary for such maintenance are called respectively the strip-in field and the collapse field. Both fields depend upon the magnetic moment per unit volume (4.pi.M.sub.s), the desired bubble size, and the thickness of the garnet layer (h).
A second magnetic field is employed to produce magnetic bubble domains and is introduced through the application of a current to a metal pattern that overlies the garnet layer. Once the magnetic bubble domains are produced, they are generally propagated through the garnet film by the use of magnetic fields induced by applying a rotating magnetic field to a permalloy pattern that overlies the garnet material or to a pattern produced through the ion implantation of a portion of the garnet material. (See T. J. Nelson, R. Wolfe, S. L. Blank, P. I. Bonyhard, W. A. Johnson, B. J. Roman, and G. P. Vella Coleiro, Bell System Technical Journal, Vol. 59, page 229 (1980) and A. J. Perneski, IEEE Transactions On Magnetics, Mag-5, 554 (1969) for a description of devices employing ion implantation and permalloy patterns, respectively.)
The properties of the device substantially depend on the intrinsic properties of the garnet layer. Intrinsic properties such as the magnetic moment, (4.pi.M.sub.s), the exchange constant (A), and the magnetic anisotropy (K.sub.u) of a garnet film strongly influence the usable bubble size for a device employing this garnet. Preferably, usable K.sub.u should be substantially growth induced. Typically a stress induced anisotropy is produced by a lattice constant, a.sub.f, mismatch between the garnet film and the substrate on which it is grown. However, the presence of strain is conducive to defect formation in the garnet film and is undesirable. Unacceptable strain is generally produced by a mismatch greater than 0.018 .ANG. in compression and 0.013 .ANG. in tension. (See S. L. Blank and J. W. Nielsen, Journal of Crystal Growth, Vol. 17, page 302 (1972). For thinner garnet layers, e.g., 1.0 .mu.m in thickness, somewhat greater compressive mismatch, is acceptable.) Indeed, in the case of extreme tensile strain, a cracked film, which is definitely outside the limits of usefulness, is produced. Therefore, the strain induced component of the K.sub.u should be limited to that corresponding to acceptable mismatches.
The smaller the bubble size the greater the information that is storable within a unit area of the garnet material. For many applications, such as digital storage, the smaller the usable bubble, the more significant the device provided the bubble mobility in the garnet is reasonably high. Thus, it is desirable to have growth induced magnetic anisotropies, magnetic moments and exchange constants that together yield bubble sizes of 1 .mu.m or less. (Bubble size is proportional to the material length parameter, l which is given by the expression ##EQU1## Both the magnetic moment and magnetic anisotropy depend on temperature. Therefore, if a device is to undergo exposure to a wide range of temperatures, its usable bubble size might, depending on the degree of temperature dependence, change significantly during fluctuation in the ambient temperature. This possibility of property change becomes significant in applications at remote locations, such as space uses. Obviously, if the supportable bubble size changes significantly in an information storage device employed in these situations, the performance of the device will degrade substantially, and thus the possibility of failure is equally substantial.
It is also advantageous in situations involving elevated temperatures that a garnet composition employed in a device have a high Curie temperature. A Curie temperature, T.sub.c, is the temperature at which the magnetic moment for all the magnetic sublattices (octahedral, tetrahedral and dodecahedral) of the garnet is zero. The magnetic properties of a garnet composition, e.g., K.sub.u and 4.pi.M.sub.s, vary drastically at temperatures approaching the Curie temperature. Thus, for stable operations, a magnetic bubble device should be operated in environments having temperatures much below the Curie temperature of its garnet film. This is particularly true of devices that employ an ion implanted layer. The ion implantation in typical doses suppresses the Curie temperature as much as 70 degrees C. (See G. P. Vella Coleiro et al, Journal of Applied Physics, 52(3), 2355 (1981).) Thus, for a device relying on implantation in the garnet active layer, the operable temperature range for a given Curie temperature is even more restricted.
Additionally, ambient temperature fluctuations also affect the magnetic properties of the permanent magnet employed in the device to maintain the magnetic bubble domains. As the field of the permanent magnet decreases with increasing temperature, the field available to support magnetic bubble domains also decreases. Therefore, the device performance degrades significantly with increasing temperature, unless the field required to maintain a uniaxial magnetic bubble domain also decreases with temperature at about the same rate as the field of the bias magnet. (Similarly, if the collapse field of the garnet decreases too quickly relative to the magnetic field of the bias magnet, domains are not maintainable.)
Typical garnet films, e.g., (YSmLuCa).sub.3 (FeGe).sub.5 O.sub.12, have desirable magnetic properties, i.e., a Ku/2.pi.M.sub.s.sup.2 of greater than 2 corresponding to a capability of supporting a bubble size of 3 .mu.m. However, although the material is desirable for a wide range of applications, its Curie temperature is relatively low--approximately 470 to 475 degrees K. Many attempts have been made to produce suitable garnet films depending on materials other than the typical (YSmLuCa).sub.3 (FeGe).sub.5 O.sub.12 garnet films. (With respect to attempts involving vanadium containing films which were not suitable for device applications, see J. Daval, J. Geynet, and D. Challeton, Conference on Magnetic Bubbles, London, U.K. September 1973, for growth of extensively cracked films and see Yu. M. Yakovlev. V. S. Filonich, M. M. Klyuchnikov, and Yu. L. Sapozhnikov, Soviet Physics Solid State, 15(5), 1077 (1973) and V. S. Filonich, Yu. M. Yakovlev, and T. A. Devyatova, Soviet Physics Solid State, 16(3), 592 (1974 ) for growth of highly strained films having essentially no growth induced anisotropy.)
Attempts have also been made to produce a suitable garnet material that has a high Curie temperature and a K.sub.u that is substantially growth induced. For example, Obokata et al, (IEEE Transactions on Magnetics, Mag-9(3), 373 (1973)) have reported attempts to grow vanadium containing films with high Curie temperatures. However, these compositions were not visible for device applications. Obokata et al did not report a Curie temperature for their films and did report that the properties of their films were inadequate for device applications. In particular, Obokata described defective films with low mobilities (160 to 170 cm/sec/Oe), and with a substantially non-linear temperature dependence of the collapse field. Additionally, the magnetic properties of the Obokata film were not appropriate to support small, 1 .mu.m diameter or less, bubble domains. Thus, although the desirability of a garnet composition with high Curie temperature is known, a composition having both high Curie temperature and the other properties desirable for use in a magnetic bubble device is not yet available.